The Internet of Things is an application area with large commercial potential as well as significant technical challenges. An example application is the monitoring and control of heating, ventilating and air conditioning (HVAC) systems at the level of individual workers. In one case, continuous capture and communication of temperature, light level and humidity allowed for fine control of environmental conditions that improved worker comfort and efficiency and reduced energy costs by 24%. Fully deploying this approach for every office worker would require hundreds of millions of sensors.
Providing power to wireless sensors is typically addressed with onboard batteries. This is acceptable for small scale deployments where a handful of sensors can be regularly serviced by a technician. However, for large commercial applications in retail stores, for which there might be hundreds of sensors per location, this maintenance quickly becomes untenable.
Next generation sensor networks may be powered by energy harvesting techniques to avoid requiring battery maintenance. Energy harvesting is a process by which energy is derived from external sources (e.g., radio frequency energy, solar power, thermal energy, wind energy, salinity gradients, or kinetic energy), captured and stored.
Energy may be harvested from radio frequency signals propagating wirelessly. With RF harvesting, wireless energy comes from a radio frequency transmitting device that is some distance away from a device that harvests energy from the radio frequency transmission. Properties of an energy harvester include its ability to harvest energy efficiently from available RF signals, its ability to store the harvested energy efficiently with minimal storage loss, and its ability to make the stored energy available to meet the voltage, current, and duty cycle requirements of a desired application.
One of the more popular forms of RF used today is Wi-Fi (also referred to as IEEE 802.11a/b/g/n etc.) communications. Today, most Wi-Fi communications are in the 2.4 GHz and 5.8 GHz frequency bands and there are many local area networks that are based on Wi-Fi in which access points enable Wi-Fi clients to gain access to networks such as the Internet. Furthermore, the 2.4 GHz and 5.8 GHz bands also support other networking standards, such as Zigbee and Bluetooth, and other proprietary networks, each transmitting energy by communicating in this same frequency band.
Recent work has shown that Wi-Fi energy is abundant in a typical office environment, although at low power levels, e.g. yielding below −20 dBm at the feedpoint of a half-wavelength, 6 dBi gain patch antenna. Harvesting energy from ambient Wi-Fi has been the subject of several recent investigations. The typical solution includes rectification of the RF power incident on an antenna into DC charge on a capacitor. Provided that power can be harvested at a rate greater than the leakage of the capacitor, eventually enough energy will be accumulated to do useful work. A particular challenge of harvesting at low power levels is the fact that the rectified energy is both power limited as well as voltage limited. This voltage limitation is significant because there is typically some minimum start-up voltage exceeding 700-800 mV for running meaningful digital circuitry, with typical commercial microprocessors requiring as much as 1.8V. An added challenge in the Wi-Fi harvesting case is the bursty nature of Wi-Fi transmissions. While a typical transmission may include millisecond-duration high energy pulses at some interval that can be stored in a capacitor, the stored energy may be consumed by circuit leakage in between bursts.
Additionally there are other frequency bands that support different communication protocols, each of which transmit energy when they are communicating. These include, for example, digital television (DTV) and Global System for Mobile Communications (GSM) signals.
Boost converters are a well-known circuit for boosting a voltage. Because the output voltage from a rectifier is usually very low, boost converters are commonly used in RF energy harvesting circuits. A common type of boost converter is a non-resonant inductor-based boost converter. In this type of DC-DC converter, a switched node draws current into an inductor from a rectifier, so that when the switch opens, voltage builds in the inductor, forward-biases a diode, and is stored at a higher voltage in the output capacitor. A major liability of this approach is the control signal for driving the switching transistor (in that case, the gate node of a MOSFET). Also the power for the MOSFET gate drive often comes from an external power supply, not the rectified RF, so it is not a fully self-powered system.
An alternative boost converter uses a self-resonant transformer based DC-DC converter driven via a JFET. This booster is both self-starting and self-oscillating, and needs no externally supplied oscillator.